In recent years, a carbon dioxide capture and storage (CCS) technology for capturing and storing carbon dioxide (CO2) has been drawing attention as a countermeasure against global warming. Specifically, there has been investigated a carbon dioxide capture system that uses an absorbing liquid to capture carbon dioxide contained in process exhaust gas (gas to be treated) discharged from a thermal power plant, a steel plant, or a garbage incineration plant.
In such a carbon dioxide capture system, the process exhaust gas is supplied to an absorber. In an absorption unit in the absorber, carbon dioxide contained in the process exhaust gas is absorbed in an absorbing liquid which contains amine and water. In this case, the process exhaust gas having released the carbon dioxide therefrom is discharged as absorption unit exhaust gas. The absorbing liquid having absorbed the carbon dioxide therein is supplied to a stripper. In a stripping unit in the stripper, the carbon dioxide is released from the absorbing liquid. The released carbon dioxide is discharged from the stripper together with steam as stripping unit exhaust gas, and the carbon dioxide is separated and captured. The absorbing liquid having released the carbon dioxide therefrom in the stripper is returned into the absorber.
Meanwhile, the absorption unit exhaust gas and the stripping unit exhaust gas can contain the amine used as a component of the absorbing liquid. As a measure to reduce the amount of the amine contained in the absorption unit exhaust gas and the stripping unit exhaust gas, it has been contemplated to cool the exhaust gases or wash the exhaust gases with water.
When the absorption unit exhaust gas or stripping unit exhaust gas is cooled, water contained in the exhaust gas is condensed to form condensed water. In this instance, the amine having been contained in the exhaust gas is absorbed in the condensed water. When the absorption unit exhaust gas or stripping unit exhaust gas is washed with water (cleaning water), the amine having been contained in the exhaust gas is absorbed in the cleaning water. Thus, in either case, the amount of the amine contained in the absorption unit exhaust gas or stripping unit exhaust gas can be reduced.
On the other hand, the condensed water or cleaning water in which the amine has been absorbed is mixed into the absorbing liquid circulating between the absorber and the stripper. By this process, the amine contained in the absorption unit exhaust gas or stripping unit exhaust gas is recovered. Accordingly, the loss of amine through discharge from the carbon dioxide capture system to the exterior of the system, called amine loss, can be suppressed.
In general, during an operation of the carbon dioxide capture system, the amount of water in the absorbing liquid circulating between the absorber and the stripper is held within a predetermined range. With the amount of water in the absorbing liquid made to be lower than a predetermined upper limit, the concentration of the absorbing liquid component is raised so as to enhance the carbon dioxide capture rate. On the other hand, when the concentration of the absorbing liquid component rises, the amount of the amine contained in the absorption unit exhaust gas or stripping unit exhaust gas can increase. In view of this, the amount of water in the absorbing liquid is set higher than a predetermined lower limit, so as to reduce the amine loss.
If the condensed water or cleaning water in which the amine has been absorbed as aforementioned is mixed into the absorbing liquid containing a large amount of water, therefore, the amount of water in the absorbing liquid can increase significantly. In such a case, it would be difficult for the amount of water in the absorbing liquid to be held within the aforementioned predetermined range.